This invention is directed generally to the field of near infrared reflectance analysis and more particularly to a novel and improved near infrared reflectance sensing system for determining soil constituents, for example, for use in agriculture or the like.
Analysis of soil constituents is of particular interest to agriculture for optimizing conditions for the raising of various crops. Heretofore such analysis was done by taking numerous soil samples from an area to be tested and subjecting the same to painstaking and time-consuming laboratory analysis.
We have proposed to greatly simplify this process by the use of a near infrared (NIR) reflectance sensing system suitable for use in the field. It has previously been proposed to use such sensing systems for other types of analysis; for example, for analysis of grain constituents or the constituent contents of other bulk materials. However, in developing a system for determining soil constituents for in-the-field use, a number of other problems and factors arise which need to be addressed.
Among soil properties of interest are soil moisture content and cation exchange capacity (CEC). However, perhaps of primary interest is the analysis of the organic carbon content of the soil. Accordingly, our sensing system is designed particularly with the analysis of organic carbon content in mind, although it might readily be adapted to analysis of such other properties as moisture content and CEC without departing from the invention. Among problems to be addressed in the design of the system were such matters as selection of design alternatives of the sample presentation mechanism, the design of the sensor and data acquisition systems and the processing and analysis of the data acquired.
The primary considerations in selection of a sample presentation mechanism were control of the moisture content and surface roughness characteristics of the sample. Control of the sample moisture content was found to be possible by sensing below the soil surface, where less variability in soil moisture would be encountered than at the surface. Control of the surface roughness characteristics of the sample was necessary, and we found this could be accomplished by a pressing, rolling, slicing, or other mechanical action. These mechanical actions would be more easily accomplished below the soil surface, where we noted a more consistently friable soil would be found. Subsurface sensing would also avoid any irregularities in sample characteristics due to the puddling or crusting which might occur on the soil surface.
Once the need to sense a subsurface soil sample was identified, three alternative means of in situ and remote sensing were investigated:
Option 1 Transport of the soil sample to a remote sensing location while maintaining the sample structure (for example, as in a soil core).
Option 2 Transport of a fractured soil sample to a remote sensing location by an auger or similar device, followed by reconsolidation of the sample for measurement.
Option 3 In situ sensing of a surface prepared by some type of furrow opener. Option 2, transport of a fractured soil sample, was eliminated from consideration due to several disadvantages. This system would have an inherent lag time, severely limiting operating speed. The process of soil detachment, transport, and repacking could introduce bias due to size and/or density sorting of the soil particles. However, option 2 did have several advantages: intermittent sampling with a sample device would be possible; the sensor optical path could be made compact; and a reflectance standard could be incorporated into the mechanism.
Option 1, transport of a consolidated soil sample, was considered in more detail. This concept used an automated device to extract a soil core and to position the core for scanning through a window in the side of the soil coring tube. A pneumatically driven core sampler was fabricated to test the soil coring concept in the laboratory. The sampler used a 150 mm stroke double-acting cylinder controlled by a four-way solenoid actuated valve connected to a 1 MPa building air supply. A 12 V time delay relay provided the control input to the solenoid, porting air to the head of the cylinder for an adjustable time interval when an input signal was applied. The relay was set such that the sampler experienced a minimum dwell time at the fully extended position and then began its return stroke, with a total cycle time of 0.4 sec. A spring-loaded pivoting break-away action was provided between the coring unit and a carrier subplate so that the corer could maintain position during the coring operation, while the carrier was moving with a horizontal velocity.
Three interchangeable soil coring tubes could be attached to the cylinder rod. These tubes provided a range of cutting and core compaction alternatives for use in varying soil conditions. Two of the tubes were standard equipment for a JMC soil sampler (Clements Associates, Inc., Newton, Iowa). The JMC "wet" sampling tube, intended for use in wetter or more cohesive soil conditions, had a long tapered cutting bit and considerable relief from the bit diameter (17 mm) to the tube diameter. The JMC "dry" tube bit was shorter and larger in diameter (19 mm) with less relief. These two tubes were fitted with an external sleeve which contained the soil core while providing a window through which the sensor could operate. The third coring tube was fabricated from 25 mm diameter steel tubing by chamfering the lower edge to create a cutting bit. No relief was provided between the bit area and the remainder of the tube.
Initial stationary tests of the coring unit were accomplished with recompacted, moist samples of Drummer Silty Clay Loam obtained at the University of Illinois Agricultural Engineering farm. No difference in core quality was observed between the two JMC bits, with both collecting acceptable samples. The straight coring tube did not obtain a satisfactory core in these conditions, due to excessive adhesion of the soil to the inner diameter of the tube.
Additional soil coring unit tests were carried out in the soil bin at the Deere and Company Technical Center, Moline, Ill. The soil used was a mixture of 40% fine river sand and 60% clay, with a moisture content of 8.5 percent. Stationary and moving tests were completed at three cone index levels, 0.5 MPa, 0.75 MPa, and 1.0 MPa. The speed limit for forming an acceptable soil core with the coring unit as tested was approximately 0.25 m/s. However, it appeared that a more refined method of holding the coring unit stationary relative to the soil surface while sampling could increase the speed operating range. Only the straight bore tube produced acceptable cores in the soil bin tests, and then only with marginal reliability. Small differences in soil moisture or cone index level resulted, on occasion, in incomplete cores being obtained. Cores collected in this high sand content, low cohesion soil with the JMC tubes fell apart easily.
Based upon the difficulties in obtaining a complete soil core reliably across a range of soil types and physical conditions, the core sampler method of sample presentation was eliminated from further consideration.
Because of the problems encountered with the remote sample presentation methods described above, it was decided to pursue in situ sensing. This method had disadvantages in difficulty of reflectance calibration and inability to hold the sample stationary while data were being acquired, but it seemed to hold the best promise for development of a workable prototype field sensor.